1. Field of the Invention
The present invention relates to a reduced or low lead aviation gasoline blend which satisfies the requirements of ASTM D 910 or deviates therefrom only in the inclusion of an oxygenate. In particular the invention relates to such an aviation gasoline blend which comprises at least about 50% by volume 2,2,4-trimethylpentane and contains less than the maximum of 0.53 mL/L of lead specified by ASTM D 910.
2. The Prior Art Background
Tetraethyl lead (TEL) was used for years to increase octane in both automotive and aviation gasoline fuels. On Jan. 1, 1996 TEL was banned from use in automotive gasoline fuels by the EPA Clean Air Act. The General Aviation industry is concerned that these criteria will be extrapolated to aviation gasoline fuels so as to increase the cost and affect the reliability and/or consistency thereof. This concern is exacerbated because aviation gasoline fuel generally must have a higher octane than automotive fuel. Moreover, consumers in the general aviation industry require a gasoline fuel product which has a consistent and reliable octane rating so as to provide an appropriate consistent performance level.
Generally speaking, aviation gasoline fuels today must meet the requirements of ASTM D 910 (1996). Presently relevant criteria of ASTM D 910 are set forth below in TABLE 1.
ASTM D 910 also excludes oxygenates. To meet the criteria of ASTM D 910, most aviation gasoline fuels today comprise a blend containing about 70 volume % alkylate, about 10 volume % isopentane and about 20 volume % toluene. Approximately 2 gm of TEL is added to each gallon of the blend to appropriately increase the MON to the specified level. This is the maximum amount of lead allowed by the ASTM D 910 specification. The very stringent specifications set by ASTM D 910 (1996) for aviation gasoline fuels, particularly the supercharge octane rating, the MON, the vapor pressure and the distillation criteria greatly reduce the options available to blend a satisfactory aviation gasoline fuel without using a lead additive to increase the octane level. In fact most prior art aviation gasoline fuel blends have necessarily included the maximum amount of lead permitted.
The ASTM D 910 (1996) specification sets only a maximum level for lead. Accordingly, aviation gasoline with no lead would be suitable assuming all other criteria are met. At this point in time the EPA is adamant about reducing or eliminating lead in aviation gasoline and it is believed that the main reason that the EPA has not yet moved on this point is that there currently is no solution to the problem of achieving high enough octane in aviation gasoline fuels without using lead. Were the EPA to move now, it might lead to the destruction of the entire general aviation industry.
So a major unsolved problem in the general aviation industry today is how to provide an aviation gasoline fuel which meets all of the criteria of the ASTM D 910 (1996) specification without using lead as an additive. Stated another way, the problem is how to reduce or eliminate the need for lead in aviation gasoline fuels.
The present invention provides a genuine solution to the problems discussed above. Thus, the invention provides a reduced lead aviation gasoline blend which satisfies the requirements of ASTM D 910. In particular, the invention provides an aviation gasoline fuel that comprises at least about 50.0 volume % 2,2,4-trimethylpentane (sometimes referred to simply as xe2x80x9cisooctanexe2x80x9d), whereby the lead content of the fuel may be reduced to less than 0.53 mL/L. Thus, the fuel of the invention may comprise at least about 50.0 volume % 2,2,4-trimethylpentane, meets the requirements of ASTM D 910, and contains less than 0.53 mL/L lead.
More particularly, the invention provides a gasoline blend which is suitable for use in an internal combustion airplane engine and which contains no more than 0.26 mL/L of lead and which may comprise from about 55.0 to about 65.0 volume % of 2,2,4-trimethylpentane. Preferably the gasoline blend of the invention may comprise from about 58.0 to about 62.0 volume % of the 2,2,4-trimethylpentane. More preferably the gasoline blend of the invention may comprise about 60.0 volume % of the 2,2,4-trimethylpentane. Ideally, the gasoline blend of the invention may comprise no more than about 85.0 volume % of the 2,2,4-trimethylpentane.
Through the use of the invention, the lead content of the gasoline blend may be reduced from the maximum allowed by ASTM D 910 to an amount which ranges from about 0.11 to about 0.15 mL/L. Preferably, and in accordance with one aspect of the invention, the lead content may range from about 0.12 to about 0.14 mL/L. Ideally, in accordance with this aspect of the invention, the lead content of the fuel may be about 0.13 mL/L. In fact, the lead content of the aviation fuel may be reduced to an amount which is no more than about 0.13 mL/L.
Through the use of another aspect of the invention, wherein the gasoline blend includes an oxygenate, the lead content of the gasoline blend may be reduced from the maximum allowed by ASTM D 910 to an amount which ranges from about 0.03 to about 0.07 mL/L. Preferably, in accordance with this aspect of the invention, the lead content may range from about 0.04 to about 0.06 mL/L. Ideally, in accordance with this aspect of the invention, the lead content of the fuel may be about 0.05 mL/L. In fact, the lead content of the aviation fuel may be reduced to an amount which is no more than about 0.05 mL/L.
A predominant feature of the invention is the use of a major amount of 2,2,4-trimethylpentane in the gasoline blend. As mentioned above, the content of this ingredient in the overall gasoline blend may be as low as 50.0% and as high as 85.0% by volume. Preferably, the blend may contain about 60.0 volume % of the 2,2,4-trimethylpentane. In accordance with another aspect of the invention the gasoline blend may contain about 67.0 volume % of said 2,2,4-trimethylpentane.
The aviation gasoline blend of the invention may also include a C7-C11 aromatic hydrocarbon, a C5-C6 aliphatic hydrocarbon and a C4 aliphatic hydrocarbon. Preferably the blend may contain a sufficient amount of a C7-C11 aromatic hydrocarbon to provide an appropriate supercharge rating. Furthermore, the blend may include a sufficient amount of a C5-C6 aliphatic hydrocarbon to provide D-86 distillation control. Additionally, the blend may include a sufficient amount of a C4 aliphatic hydrocarbon to provide D-86 distillation and RVP control.
In one highly preferred embodiment of the invention, the aviation gasoline blend may comprise from about 56.0 to about 64.0 volume % 2,2,4-trimethylpentane, from about 13.0 to about 17.0 volume % xylene, from about 12.0 to about 16.0 volume % methyl t-butyl ether, from about 6.5 to about 9.5 volume % isopentane, from about 2.0 to about 4.0 volume % isobutane and no more than about 0.1 mL/L tetraethyl lead. More preferably, the aviation gasoline blend of the invention may comprise from about 58.0 to about 62.0 volume % 2,2,4-trimethylpentane, from about 14.0 to about 16.0 volume % xylene, from about 13.0 to about 15.0 volume % methyl t-butyl ether, from about 7.0 to about 9.0 volume % isopentane, from about 2.5 to about 3.5 volume % isobutane and no more than about 0.07 mL/L tetraethyl lead. Ideally the blend may include about 60.0 volume % 2,2,4-trimethylpentane, about 15.0 volume % xylene, about 14.0 volume % methyl t-butyl ether, about 8.0 volume % isopentane, about 3.0 volume % isobutane and no more than about 0.05 mL/L tetraethyl lead.
In another highly preferred embodiment of the invention, the aviation gasoline blend may comprise from about 62.0 to about 72.0 volume % 2,2,4-trimethylpentane, from about 15.0 to about 21.0 volume % xylene, from about 10.0 to about 14.0 volume % isopentane, from about 2.0 to about 4.0 volume % isobutane and no more than about 0.20 mL/L tetraethyl lead. More preferably, the aviation gasoline blend of this embodiment may comprise from about 64.0 to about 70.0 volume % 2,2,4-trimethylpentane, from about 16.0 to about 20.0 volume % xylene, from about 10.5 to about 13.5 volume % isopentane, from about 2.5 to about 3.5 volume % isobutane and no more than about 0.20 mL/L tetraethyl lead. Ideally, the blend of this embodiment may include about 67.0 volume % 2,2,4-trimethylpentane, about 18.0 volume % xylene, about 12.0 volume % isopentane, about 3.0 volume % isobutane and no more than about 0.13 mL/L tetraethyl lead.
In a broader sense, and in one preferred aspect of the invention, the aviation gasoline blend of the invention may include from about 56.0 to about 64.0, more preferably from about 58.0 to about 62.0 and ideally about 60.0 volume % 2,2,4-trimethylpentane, from about 13.0 to about 17.0, more preferably from about 14.0 to about 16.0 and ideally about 15.0 volume % of a C7-C11 aromatic hydrocarbon, from about 12.0 to about 16.0, more preferably from about 13.0 to about 15.0 and ideally about 14.0 volume % of an oxygenate, from about 6.0 to about 10.0, more preferably from about 7.0 to about 9.0 and ideally about 8.0 volume % of a branched aliphatic C5-C6 hydrocarbon, from about 2.0 to about 4.0, more preferably from about 2.5 to about 3.5 and ideally about 3.0 volume % of a branched C4 aliphatic hydrocarbon and no more than about 0.1 mL/L tetraethyl lead.
In another preferred aspect of the broader invention, the aviation gasoline blend of the invention may include from about 63.0 to about 70.0, more preferably from about 65.0 to about 69.0 and ideally about 67.0 volume % 2,2,4-trimethylpentane, from about 15.0 to about 21.0, more preferably from about 16.5 to about 19.5 and ideally about 17.0 volume % of a C7-C11 aromatic hydrocarbon, from about 10.0 to about 14.0, more preferably from about 11 to about 13 and ideally about 12 volume % of a branched aliphatic C5-C6 hydrocarbon, from about 2.0 to about 4.0, more preferably from about 2.5 to about 3.5 and ideally about 3.0 volume % of a branched C4 aliphatic hydrocarbon and no more than about 0.2 mL/L tetraethyl lead.
In a larger, broader sense, the invention provides an aviation gasoline blend which satisfies the requirements of ASTM D 910, contains no more than 0.26 mL/L of lead and comprises a sufficient amount of 2,2,4-trimethylpentane to provide the blend with a motor octane number of at least 99.5.
The invention provides an aviation gasoline fuel blend which includes a reduced amount of lead and still complies with all of the criteria set by the ASTM D 910 specification for aviation fuel. These criteria of ASTM D 910 are set forth above in TABLE 1. In accordance with the invention, the principal component of the aviation gasoline of the invention is 2,2,4-trimethylpentane, a C8 branched chain aliphatic hydrocarbon that is sometimes referred to as isooctane. Preferably the isooctane is present in the fuel of the invention in an amount which is at least 50.0 volume % of the entire blend. Isooctane may be present in any amount up to and including about 85.0 volume % . When isooctane is present in an amount less than about 50% by volume, the octane will generally be too low. When the amount of isooctane exceeds 85.0% , the vapor pressure will be to low and the distillation profile will not meet the ASTM D 910 criteria.
In one preferred embodiment, the aviation gasoline blend of the invention may desirably include from about 62.0 to about 72.0 volume % 2,2,4-trimethylpentane, from about 15.0 to about 21.0 volume % of a C7-C11 aromatic hydrocarbon, from about 10.0 to about 14.0 volume % of a branched aliphatic C5-C6 hydrocarbon, from about 2.0 to about 4.0 volume % of a branched C4 aliphatic hydrocarbon, and no more than about 0.26 mL/L tetraethyl lead.
In another preferred embodiment, the aviation gasoline blend of the invention may desirably include from about 64.0 to about 70.0 volume % 2,2,4-trimethylpentane, from about 16.0 to about 20.0 volume % of a C7-C11 aromatic hydrocarbon, from about 10.5 to about 13.5 volume % of a branched aliphatic C5-C6 hydrocarbon, from about 2.5 to about 3.5 volume % of a branched C4 aliphatic hydrocarbon, and no more than about 0.20 mL/L tetraethyl lead.
In yet another preferred embodiment, the aviation gasoline blend of the invention may desirably include about 67.0 volume % 2,2,4-trimethylpentane, about 18.0 volume % of a C7-C11 aromatic hydrocarbon, about 12.0 volume % of a branched aliphatic C5-C6 hydrocarbon, about 3.0 volume % of a branched C4 aliphatic hydrocarbon, and no more than about 0.13 mL/L tetraethyl lead.
In a further preferred embodiment, the aviation gasoline blend of the invention may desirably include from about 56.0 to about 64.0 volume % 2,2,4-trimethylpentane, from about 13.0 to about 17.0 volume % of a C7-C11 aromatic hydrocarbon, from about 12.0 to about 16.0 volume % of an oxygenate, from about 6.5 to about 9.5 volume % of a branched aliphatic C5-C6 hydrocarbon, from about 2.0 to about 4.0 volume % of a branched C4 aliphatic hydrocarbon, and no more than about 0.1 mL/L tetraethyl lead.
In a still further preferred embodiment, the aviation gasoline blend of the invention may desirably include from about 58.0 to about 62.0 volume % 2,2,4-trimethylpentane, from about 14.0 to about 16.0 volume % of a C7-C11 aromatic hydrocarbon, from about 13.0 to about 15.0 volume % of an oxygenate, from about 7.0 to about 9.0 volume % of a branched aliphatic C5-C6 hydrocarbon, from about 2.5 to about 3.5 volume % of a branched C4 aliphatic hydrocarbon, and no more than about 0.07 mL/L tetraethyl lead.
In an additional preferred embodiment, the aviation gasoline blend of the invention may desirably include from about 58.0 to about 62.0 volume % 2,2,4-trimethylpentane, from about 14.0 to about 16.0 volume % of a C7-C11 aromatic hydrocarbon, from about 13.0 to about 15.0 volume % ofan oxygenate, from about 7.0 to about 9.0 volume % of a branched aliphatic C5-C6hydrocarbon, from about 2.5 to about 3.5 volume % of a branched C4 aliphatic hydrocarbon, and no more than about 0.1 mL/L tetraethyl lead.
As is well known to those of ordinary skill in the art which pertains to the present invention, the blending of useful gasoline fuels, for either automobile or aviation use, is more an art than a science. Each component may provide a specific property or characteristic; however, when such a component is added to a blend it may tend to alter other necessary properties and characteristics. Accordingly, much trial and error is necessarily involved in order to achieve a blend which meets all of the criteria which have been preordained, such as, for example, by the ASTM D 910 specification. Thus, when one adds a component such as isooctane for the purpose of achieving a higher octane, the overall result may be that the vapor pressure becomes too low for the blend to be useful.
Although prior workers in the field have tried alkylates which contain a substantial isooctane portion, none have previously been successful. For example, SAE Technical Paper No. 971490 of J. N. Valentine et al., entitled xe2x80x9cDeveloping a High Octane Unleaded Aviation Gasoline,xe2x80x9d presented during the General, Corporate and Regional Aviation Meeting and Exposition held at Wichita, Kans. on April 29 through May 1, 1999, describes certain experimental activities involving the testing of blends containing xe2x80x9ca wide boiling range alkylatexe2x80x9d in combination with an xe2x80x9coctane boostingxe2x80x9d component such as methyl-tertiary-butyl ether (MTBE) or ethyl-tertiary-butyl ether (ETBE). The wide boiling range alkylate included isooctane but the proportion thereof in the alkylate is not discussed. The final conclusion set forth by the authors was that xe2x80x9cthe experimental fuels blended for this program are not ready to be used in aircraft.xe2x80x9d
In accordance with one aspect of the present invention, the low lead aviation gasoline fuel blends provided thereby comply fully with all of the requirements of ASTM D 910. In accordance with another aspect of the invention, the low lead gasoline fuel blends incorporate an oxygenate, but otherwise comply fully with all of the requirements of ASTM D 910. In both cases these blends are ready to be used in aircraft.
In addition to the isooctane, the aviation gasoline blends of the invention preferably may include an aromatic component to increase supercharge rating and one or more lighter aliphatic hydrocarbons for distillation profile and RVP (Reid Vapor Pressure) control. The aromatic component may be a C7-C11 aromatic hydrocarbon and preferably may be xylene or toluene. The aliphatic hydrocarbon components ideally may be branched compounds and preferably may have 4 to 7 carbon atoms in their chains. Preferably the aliphatic hydrocarbon components may include a mixture of isopentane and isobutane. The isopentane primarily is useful for controlling the distillation profile while the isobutane is useful for adjusting both distillation profile and vapor pressure.